I would say that 习惯 is "being used to sth" naturally. So I would say it's a condition.
适应 means "to adapt, to become accostumed at sth. that you weren't used to before". So I would say it's a change of condition.

@Tarlou, Youngfun, Lucia_zwl
Thank you all for your helpful replies! 吃得惯/吃不惯 is exactly what I was looking for (an expression I had forgotten--and it looks like I have lots of other good alternatives to choose from too)!